A. Technical Field
This invention relates to the electrocardiogram (ECG), and, more particularly, to non-invasive monitoring and detection of risk patterns in the ECG attributable to medication or physical activity.
B. Prior Art
Cardiovascular diseases are pervasive, contributing to over 2.4 million deaths annually in the United States alone. Delay in recognition and treatment of a heart abnormality leads to more damage to the heart, higher cost of hospitalization and lower quality of life for the survivors.
Heart abnormalities typically develop over time and the risk of a heart attack can increase with adverse side effect of certain medications. Drugs affecting the heart, referred to herein collectively as cardioactive drugs, may be targeted for cardiovascular disease or other ailments not related to the heart. Anti-arrhythmia drugs are examples of drugs targeting the heart for the control of irregular heartbeats. Vioxx® and Celebrex®, which were widely prescribed for arthritis patients, are examples of cardioactive drugs not targeting the heart.
Pharmacological therapy represents the first line of defense for most cardiac abnormalities. For atrial fibrillation (AF), a common cardiac abnormality affecting millions of people, the medication can be targeted for slowing the conduction of electrical impulses, decreasing the excitability and automaticity of cardiac cells, or prolonging the refractory period of cardiac tissue. The effectiveness and tolerance of these medications are quite individualized. Medications are often changed in order to achieve the desired outcome of reducing symptomatic episodes of AF. To further complicate matters, some of these drugs can actually have the opposite effect causing the heart to become more irritable and setting the stage for new arrhythmias to occur.
Control of the ventricular rate due to AF is also important in that a prolonged rapid heart rate can cause permanent physiologic damage to the cardiac cells. These cells can undergo a form of “remodeling” that reduce the contractility of the heart muscle and cause cardiomyopathy to develop. It can be very challenging for the cardiologist to achieve adequate ventricular rate control in some patients. In such cases, the patient's heart rate may be well controlled with medication while at rest, but quickly exceeds the desired range as the patient becomes moderately active. Conversely, prescribed medication may control the patient's heart rate during activity but can cause the heart to slow excessively when the patient is at rest. Fine-tuning the medication regimen is important for reducing patient discomfort and minimizing adverse physiologic changes. Besides controlling heart rate, other medication can cause subtle changes in ECG patterns leading to arrhythmia and event heart attacks for certain individuals. ECG symptoms for increased cardiac risk include prolongation of QT interval and ST segment shifts.
By continuously monitoring the heart rate and detecting ECG patterns for an individual over time, the effects of cardioactive medication can be studied. Corrective measures can be taken prior to developing a serious cardiac condition. Unfortunately, continuous monitoring and detection of ECG risk patterns is not feasible with standard “resting ECG” instruments available in clinical setting. Visual observation of standard ECG charts is not likely to reveal trends and subtle changes in ECG patterns.
It is well known in the field of cardiovascular science, that a single ECG recording is often non-diagnostic for many individuals, including those with serious heart disease such as acute myocardial infarction. These individuals often exhibit “normal” ECG patterns, even during a cardiac episode. However, risk patterns can be revealed if the ECG is compared with previously recorded ECG patterns. Furthermore, certain individuals with an apparent abnormality in their ECG may actually have normal cardiac function if their ECG pattern is consistent over time. This is especially the case when age is considered in the diagnosis. For these and other reasons, differential ECG measurements taken over time with consecutive ECG readings are considered essential for the cardiac diagnosis of certain individuals. A differential measurement not only reveals variations in cardiac rhythms, but also shifts and trends in the ECG waveform patterns. Detection of these changes often requires the aid of a processor (a computer, a microprocessor or a digital signal processor). Using a processor, signal averaging techniques can be used to average a specific time window of the periodic ECG waveform for noise reduction and detection of ECG segment baseline shifts over time.
Microprocessor based ambulatory monitors have been developed to solve some the limitations of large hospital-based ECG instruments. For example, Holter monitors are portable ECG used mostly at home to monitor the ECG of an individual. These instruments typically use 5 or more ECG electrodes attached to the chest at one end and connected to a portable electronic device at the other end. The device is worn or strapped to the body for recording ECG signals in its memory. After 24 to 48 hours of continuous monitoring and recording, the Holter monitor is typically returned to the clinic, where the recorded ECG data is downloaded for review, record keeping, and for further analysis by clinicians.
Cardiac event monitors are similar to Holter monitors but generally smaller and have less memory for recording only a few minutes of ECG during a cardiac event. They are designed to detect an intermittent cardiac event, i.e. heart palpitation, dizziness, syncope, chest pain, etc. The looping memory event recorder is a more sophisticated version of an event recorder with a miniature electronic package attached to the patient's chest via two to three electrodes. Event monitors typically record a short segment of the ECG prior to activation by a switch. For example, when a patient experiences a palpitation, the device records the prior 45 seconds of ECG and also 15 seconds subsequent to switch activation. With this method of monitoring, transitory cardiac symptoms can be documented.
Although less bulky than Holter monitors, event recorders are also uncomfortable to wear and lack the diagnostic capabilities of Holter monitors. The ECG patches (electrodes) used with such prior art ECG monitors are disposable and replaced frequently for extended monitoring. However, the base unit of these instruments is reusable as it is “loaned” to patients by the clinic providing the diagnostic service.
The prior art instruments and methods discussed above, and others discussed below, fall short of providing inexpensive and comfortable means for detection of cardiac abnormalities and trends developing over time. Furthermore, prior art instruments do not provide the ability to select monitoring parameters and detection criteria, which need to be customized for the individual or a patient group. These settings often need to be individualized according to the age, cardiac condition, medication used and the activity level permitted for the individual.
U.S. patent application serial no. 2003/0069510 to Semler discloses a disposable vital signs monitor that is a “flexible, nominally flat planar form having integral gel electrodes, a sticky-back rear surface, an internal flex circuit capable of sensing, recording, and play out several minutes of the most recently acquired ECG waveform data and a front surface that includes an output port preferably having one or more snap connectors compatible with lead harness . . . . ” The monitor is designed for short-term monitoring as stated: “a relatively short term battery life, as it is intended for limited-term use.” Semler's invention is primarily an ECG recorder and it is neither programmable nor designed to detect ECG risk patterns intrinsically. This and other limitations as disclosed exclude Semler's invention for conveniently monitoring the effect of medications or stresses on the heart, particularly in home settings.
U.S. Pat. No. 5,634,468 to Platt et al. discloses a sensor patch for obtaining physiologic data, including temperature, and transmitting a conditioned signal to a portable unit nearby and subsequently to remote monitoring equipment. Platt's patch neither saves ECG data nor performs analysis. For these purposes, it relies on external devices.
Recent publicity regarding adverse effects of certain medications highlights the need for more effective means of monitoring the effects of cardioactive drugs, including during clinical trials prior to regulatory approval. Using existing instruments to continuously monitor the ECG for a large patient population over an extended period of time is extremely costly and prohibitive in most cases.
Another problem with current heart monitoring is related to implant pacemakers and defibrillators. These programmable implant devices sometimes fail to properly deliver the necessary stimuli to the heart due to variety of reasons including incorrect device programming or improper lead attachment. Providing non-invasive ECG monitoring with selectable criteria for monitoring and setting off an alarm is highly desirable for millions of implant wearers, particularly new users.
A major objective of this invention is to provide an inexpensive non-invasive device and method to monitor the effect of drugs over an extended period of time. This device must be convenient, unobtrusive with minimal impact on the lifestyle of the user to ensure compliance with long term monitoring.
Another objective of this invention is to provide a highly miniaturized body-worn ECG monitor for automatically detecting subtle cardiac shifts.
Furthermore, a major objective is to provide ECG monitoring with means for individually selecting the monitoring mode, detection parameters and alarm criteria.
Another objective is to provide a programmable ECG sensor with automatic detection and alarm for alerting the patient to a cardiac stress condition according to programmed criteria.